Electrical measuring instrument



Feb. 5, 1946. D s vER ETAL 7 2,394,113

ELEC'ITRICALMEASURING INSTRUMENT Filed July 24, 1944 2 Sheets-Sheet 1 Fig.2.

1 I I EJ "I: r-% M5 ..4 II I 5' E? 7 I I I I |I T u r. r: y

I Inventors: John D. Seaver, Enoch M. Frge,

ir Attorneg.

1946- J. D. sEAvER ETAL ELECTRICAL MEASURING INSTRUMENT Filed July 24, 1944 2 sheets-sheet 2 Hm lIHIl I Inventors: John D. Seaver,

Enoch M. Frye, flaw/1761M Their Attorney.

support for the armature assembly.

Patented ch. 5, 1946 UNITED sra'rss PATENT o FFICE ELECTRICAL MEASURING INSTRUMENT John D. Seaver, Marblehead, and Enoch M. Frye,

Melrose Masa, assignors to General Electric Company, a. corporation of New York Application July 24, 1944, Serial No. 546,329

6 Claims. (Cl. 171-85) Our invention relates to electrical measuring instruments of the stationary permanent magnet, direct current type, and the object of our invention is to provide a small, low cost, easily assem: bled instrument of rigid construction which at the same time has a long scale, good sensitivity, and high efiiciency, especially with respect to the use of the permanent magnetic material therein.

In carrying our invention into efiect, we employ a C-shaped radially polarized magnet cast on an inner pole piece shoe, and an outer shield and flux return yoke member is then cast on the magnet. The outer yoke member has a tongue part projecting inwardly in the opening of the C-shaped permanent magnet which supports an inner hollow magnetic core concentric with the inner pole shoe. Between the core and its support is clamped a magnetic bracket forming a The features of our invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of our invention, reference is made in the following description to the accompanying drawings in which Figs. 1 and 2 represent plan and sectional side views of a permanent magnet assembly used in our instrument; Fig. 3 shows a keeper employed to preserve the permanent magnet strength of the meter magnet during assembly; Fig. 4 is an exploded view of parts for supporting and associated with the armature; Fig. 5 is a sectioned side view of .the assembled instrument; Fig. 6 is a detail end view of the armature shaft to represent how the armature coil is fastened thereto; and Fig. 'I shows a modified form of an armature supporting top plate that may be used for long pointer instruments.

Referring to Figs. 1 and 2, I represents an inner pole shoe of C shape and which can be a section of magnetic tubing with a sector removed. Preferably, the piece 9 is knurled on its outer surface. About the piece I is cast a C-shaped part 3 made of permanent magnetic material. The axial length of the part I is somewhat longer than the casting 3 as shown in Fig. 2. The outer surface of casting 3 is then grit blasted to remove excess sand and to leave the peripheral surface rough. The parts I and 3 are then placed in a mold and the outer yoke or pole shoe part 4 cast thereon. It is noted that the part 311% a reduced peripheral depth and that the abutting surface of part 4 is channel shaped and is somewhat deeper than part 3, so that in the operation v vening air gaps.

of casting part 4 to part 3, the material of part 4 fiows around the outer edge-of part 3 on three sides and grasps the part 3 so that there is no possibility of its breaking loose. In the two casting operations, first casting the magnet part 3 on part I and second casting part 4, on part 3 is' accompanied by a certain amount of shrinkage of the material being cast so that the bond or joint between parts I and 3 and between parts 3 and 4 is very tight; the materials fitting together and filling up any irregularities with negligible inter- The material used for casting the yoke part 4 is magnetic such as low carbon iron, a silicon steel, or a magnetic alloy of cobalt and iron to improve the flux carrying capacity thereof; or it is possible that in some cases part 4 might be of a permanent magnet nature in which case, it would contribute some magnetomotive force to the magnetic system. Part 4 has a tongue 5 extending inwardly between and spaced from the open ends of the C-shaped parts I and 3 and is an integral part of the casting 4.

The surfaces of parts I, 3, and 4 shown uppermost are fiush with each other. This is accomplished approximately in the casting operation and the upper surface may then be ground fiat if desired. The extended lower surfaces of parts I and 4 as shown in Fig. 2 are preferably machined finished even with each other and parallel with the upper surface. The outer peripheral surface of part 4 may be turned finished if desired, although the casting finish of this surface may be sumciently true and smooth as to fit into a cylindrical casing without further attention. The

inner peripheral surfaces of parts I and 5 are" bored out simultaneously to the same radius,

and hence their surfaces conform to spaced sector.

and threaded as needed. It is to be noted here that all of these bolt or screw openings are made in the softer magnetic material part 4 and not in the hard permanent magnet part 3. The permanent magnet part 3 itself is very snugly grasped by the casting 4 and requires no further clamping or fastening features. In making provision for the hole at 6, the magnet part 3 may be notched at l4 and the material of art 4 allowed to run into this notch adjacent the point I! where hole 6 is to bebored. Hence, drilling into the hard permanent magnetic material 3 is avoided. I It-is noted that hole comes at a natural dividing point for the flux in the structure where there is minimum flux density.

The construction also prevents any possibility of turning oi. the part 3 relative to part 4 of the magnet thus formed.

After the necessary drilling and other machining operations the structure is ready to be permanently magnetized, which is accomplished so 1 that the material 3 is polarized in a radial direction as indicated by the radial lines designated -land in Fig. 1. Before the polarizing force is completely removed, we prefer to place a C-shaped magnetic keeper, Fig. 3, across thepro-j Jecting'surfaces of parts I and 4,- and extending around the structure opposite part 3, to maintain the polarizing strength of the magnet 3 at a high value until the inner core part is assembled in place. The shape of a suitable keeper for this purpose is shown in Fig. 3 but reduced to one- I the assembly of other instrument parts as will be explained. s

The armature assembly parts, but without the armature itself, are represented in exploded view in Fig. 4, and a cross-sectional view of the parts with the armature as assembled in the magnet of-Figs. .1 and 2 is shown in F18- 5. The armature assembly framework is preferably made in two parts I5 and It. The part I5 is made of magnetic material and has the upright portion ll of sector shaped cross section which fits between the inner end of tongue 5 on the magnet and the central magnetic core part l8 when the instrument is assembled. There are an inwardly extending lower part is for supporting the lower bearing and an outwardly extendingupper part 20 having openings 9a and Illa which will register with openings 5 and It in the pole shoe, respectively, of Fig. 1 when the instrument is assembled. There are downward extending bushings 2! about the openings 8a and Illa with finished lower surfaces to set against the top surface of the magnet when assembled. A central opening at 22 in part 25 accommodates the bushing 23 of a zero lows:

set lever 24 when assembled in the form of the invention represented in Fig. 5. The armature assembly frame part It is preferably made of nonmagnetic material. It is generally triangular in shape with the inwardly extending tongue for accommodating the top bearing screw in a threaded opening at 25, and laterally extending parts having openings at .25 and 21 for securing a scale plate 2-8. Openings 91), 10b, and 2217 are provided to register with the openings9a, Illa, and 22 in frame part 20 when assembled.

The upper and lower bearing screws 29 and 29" are screwed into the openings at 25 and in the arm l3 provided therefor, 35 are lock nuts for these screws. Between-the lock nuts and the surface of the supporting framework upp r and lower spiral supports 3| and 32 are clamped with ,friction washers 33. The inner end of zero set lever 24 engages a projection on the upper spiral support 3| so that the upper spiral spring 34 may be adjusted for setting the pointer on zero after assembly. A screw, not shown, enters through the openings 5b, 8a into the opening 3 which is threaded, and another screw enters through the openin s lob. Illa, into 10 which is threaded to assist in securing the parts in assembled relation;

The armature comprises a shaft 35 having pivots in its upper and lower ends engaging jewels in the jewel screws 29 and 25"in the usual manner. To the shaft are secured the armature frame spool shell 35a and coil 35, the spiral lead-in conductors 34 and 31, and the pointer 38 with balanc. ing arm and counterweight 33. The shaft 35 has extruded rectangular shaped enlargements 35c -(see Fig. 6) to which the sides of the spool shell 35a are welded. The armature winding 35 is connected to the spirals by suitable connections in a usual manner. There is a central opening in the scale plate 2-3 through which the pointer extends and through which there is access to the top bearing. parts. For appearances sake this center opening may be masked by a part 39. Three studs, one of which is shown at 40, enter through the openings 5, I, and 8 of the magnet to hold it in spaced relation on a base, 4|. Spacer bushing 42 is provided to support the top edge of the scale plate 28.

The design is intended to be assembled as fol- The armature is first assembled in the armature assembly frame. This includes the parts shown in Fig. 4 with the exception of core parts It and 44. Next the core part it is added by passing the armature shaft 35 and adjacent side of coil 35 through the opening 45 shown in one side of core'part l8. Core locking piece 44 of T-shaped cross section, and which is made of magnetic material, is then inserted endwise in the T-shaped opening 45, and this piece with the I core is turned to face toward the sector shaped vertical part I! of frame piece l5, with the lower screw holes 45 and 41 in these parts in alignment.

The screw 48 (see Fig. 5) is then used to secure.

these core parts ls-and 4-5 to the part II, the screw being tightened to draw core l3 tightlyto part ll, This is facilitated by reason of the fact that the T-shaped locking part 44 has a radial thickness ,which permits it to be spaced slightly from part 11 when the core part It is tight against it as shown in Fig. 5. In assembling these core parts the core part it is positioned vertically by having its lower edge rest upon the upper surfaces of-proiections 43 on part II.

The armature and core assembly is then inserted from the top into the magnet of Fig. 1 with the outer surface of assembly frame part ll, against the inner surface of tongue 5, with the upper screw hole 50 of locking part 44 in align ment with the opening, 5| in part II and hole II in the pole shoe. material (see Fig. 5), is then added and tightened, locking the ,core' parts and armature assembly to the magnet with the armature assembly framework part I! clamped between the care it and tongue 5 of magnet part 4. The abutting surfaces of these parts are finished to fit perfectly to provide a low reluctance flux pathradially between core 13 and tongue 5, and exactly to center the cylindrical core part It in the circle formed by the inner surfaces of inner pole shoe l and tongue5 of the pole shoe of Fig. 1. It is now seen why the part ,I1 is made of magnetic material, as it serves both as part of the armature assembly framework and as a part of the permanent magnet flux circuit of the instrument. The making of screws 48 and 52 and of locking head 44 of magnetic material contributes to the low reluctance of and the high utilization of the material in this fluxpath where magnet flux returns from the outer pole piece Serew 52. made of magnetic 1 the permanent also pa s from the outer part of the tongue I- use the same instrument element with a much,

through parts 2| andfl' to the lnnercore. This is not important but to the extent which this oc-' curs it is beneficial.

During the assembly of the inner core part and armature assembly framework to the magnet as justdescrib-ed, the keeper ofFigQ3 iskept on the bottom surface of the magnet, the opening I! in such keeper being opposite tonguepart. 5 so this assembly can proceed without interference by reason of the presence of the keeper. 'After aslonger pointer in which case, or in any case, it may be desirable to have the pointer extend in a direction from the shaft 35 opposite to that of the armature coil 38 in order to have the armature and coil tend to balance each other. such an arrangement we substitute for the top assembly plate I6 of Figs. 4 and 5- the' plate sembly then the'keeper is removed, since now the magnet circuit is completed except for the circular armature air gap between core l8 and inner pole shoe I, and has the same reluctance as it will have in service.

It will be evident that if after initial magnetization and before the assembly of core I 8 in the permanent magnet structure as above described, the magnet were allowed to become open circuited, so to speak, by omitting the use of the keeper, its magnetic circuit would be subject to a much higher reluctance condition than exists in use after the core I8 is assembled therein and, hence, would be knocked down or would lose a substantial portion of its useful flux strength. By means of our construction which permits the use of the keeper of Fig. 3 without interference during assembly, it is possible to use a smaller permanent magnet for a given useful permanent magnet flux strength in the completed instrument than would otherwise be possible, as it is not feasible to magnetize the permanent magnet of the instrument after assembly. The assembly of the armature and core I 8 complete, without the necessity of the permanent magnet structure being present, is also desirable from the point of vview of keeping the permanent magnet circuit clean from magnetic dust particles that cause trouble.

Owing to the shrink fit between the outer and inner pole shoes of the permanent magnet and the permanent magnet itself obtained by the easting procedure hereinbefore described, and the feature of protecting the magnet against demagnetization during assembly, a permanent magnet instrument of exceptionally small size as compared to its useful permanent magnet flux becomes possible. Instruments embodyingthese features have been designed and successfully tested, having an outside diameter of the permashown in Fig. 7 at lia. Plate lGa hasflthe openings 9b and lllb for alignment with 8a and Ila of assembly frame part 20 ofFig. 4.in order to secure these part together by suitable rivets or screws. The plate 18o hascircular'arms ll' extending around to the other side of the top hearing and a. slightly raised inwardly projecting tongue 55 for supporting the top bearlngsorew 29.

This allows the pointer 38a to be brought out on the side opposite from thearmature coil 36 as in dicated. The pointer 38a may be double the length of that shown inFig. 5, and the scaleplate on which it indicates is of large diameter and, hence, is preferably supported by means other than on the top assembly plate I611.

In accordance with the provisions of .thepatent statutes we have described the principle of operation of our invention, together with the apparatus which we now consider to represent the best embodiment thereof, but we desire to have it nent magnet structure of Fig. 1 of 2 inches and closure in a casing not exceeding three inches in a diameter. The construction is simple and rugged. It will be understood that the permanent magnet produces a radial flux across the concentric air gap between the inner pole shoe l and the hollow cylindrical core it; that this flux has a substantially uniform distribution about the gap and returns to the outer shoe part 4 through the tongue 5; and that when direct current flows in the armature coil, the ar'mature deflects against the. tension of the spiral springs 31 and 34, producing a measurement deflection.

The instrument illustrated in Fig. 5 has a pointer 38 of a length approximately equal 'to the radius of the maximum radius of the magnet, and the outer diameter of the scale and magnet are approximately the same. It is also possible to understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. In an instrument, a permanent magnet assembly comprising a circular tubular inner pole shoe of magnetic material, an outer circular pole shoe of magnetic material and a circular intermediate portion of permanent magnet material, the inner and intermediate portions having an open sector and the outer portion having a tongue projecting inwardly into such open sector in spaced relation to the sectored parts, said assembly being formed by first casting the permanent magnet material about the inner tubular pole shoe and second casting the outer pole shoe about the permanent magnet material whereby the contacting surfaces between permanent magnet material and pole shoes are intimately united by a shrink fit incident to the casting operations, said permanent magnetic material being polarized radially, and a cylindrical core of magnetic material positioned in spaced relation to and concentrically within the tubular inner shoe and magnetically and mechanically connected to the inner surface of said tongue portion.

2. In an instrument, a permanent magnet magnetic fiux path assembly comprising an inner circular tubular pole shoe of magnetic material, an outer circular pole shoe or magnetic material having a channeled inner periphery, an intermediate circular portion of permanent magnetic material extending betweensaid pole shoes and polarized radially, said inner and intermediate portions having an open sector and said outer portion having a tongue projecting inwardly into such open sector in spaced relation to the sectored portions, said assembly being formed by first casting the permanent magnetic material For j- "said different portions end of the assembly and the surfaces of the pole shoe portions extendin slightly beyond the per- -manent magnet portion 1 at i circular core of magnetic material magnetically and-mechanically connected to the'inner surface of said tongue and within and in spaced relation tosaid inner tububeing flush at one axial the other end, and a positioned concentrically lar pole shoe, a

3. In an electrical instrument of the direct current armature permanent magnet field type, a neidmagnet having a c-shaped magnetic part,

and a circular outer magnetic part united to the outer-surface of, the O-shaped part and with a tongue of magnetic material extending inwardly within the opening in the spaced relation to the sides, of such opening, a cylindrical'core of magnetic material positioned concentrically within and in spaced-relation to the c-shaped part, and an armature assembly framework having a magnetic part clamped between the inner surface of said tongue and said from theouter circular part within the opening inthe c-shaped part and in spaced relation to the sides of such opening, a hollow cylindrical core part positioned concentrically within the O-shapedpart and separated therefrom by an air gap, and a sector shaped part clamped be tween and magnetically joining the hollow cylindrical part and the inner surface of said tongue, said magnetic circuit containing permanent magthe sector shaped portion of said magnetic circuit. I 4

5'. An electrical measuring instrument compris- 1 ing a G-shaped permanent magnet polarized in a radial direction and having inner andouter pole shoes of magnetic material, the outer pole shoe being in the form of an annular ring with a tongue part extending inwardl in the G-shaped opening of the permanent magnet but spaced therefrom, the inner peripheries of the inner pole shoe. and tongue being shaped to conform to' spaced sector portions of the same circular cylindrical'surface, a hollow cylindrical core. of magr netic material positioned concentrically within' C-shaped part and in and in spaced relation to the inner periphery of the inner pole shoe defining an air gap between them, an armature assembly framework for rotatively gap having portions extending beyond the ends ofsaid core and 'a connecting portion of sector shaped cross section made of magnetic material clamped between said core and tongue and having a radial thickness conforming to the radial dimension of said air gap.

6. An electrical measuring instrument having a magnet assembly comprising a O-shaped permanent magnet polarized in a radial direction "and having an outer pole shoe in the form of a continuous ring with an inwardly projecting I tongue portion extending within the opening between the limbs of the C-shaped permanent magnet and in spaced relation thereto, the permanent magnet and tongue portions having inner pcripheral surfaces conforming to spaced sector portions of the same circular cylindrical surface, an armature assembly framework having a Dortion of sector shaped cross section and made of net material and being polarized to produce a radial flux across said air gap and through the,

c-shaped part with a flux return path through the tongue and sector shaped part to the cylindrical core, an armature having a shaft extending magnetic material which is adaptedto be removably secured to the inner peripheral surface of said tongue, a hollow cylindrical core having a removable locking portion in one side for the purpose of removably securing said core to the sector shaped framework and concentrically to position the core within the inner periphery of the C-shaped permanent magnet, an armature have ing a shaft and a coil rotatively supported by said framework, the shaft and one coil side being dimensioned to be inserted through the opening in said core when the locking portion thereof is removed, said framework with the core and armature assembled thereon being axially removable from one end of said magnet assembly.

JOHN D, SEAVER. ENOCH M. F'RYE.

supporting an armature coil in said air 7 Patent No. 2,394,113.

' "ca-name o! CorretTliar Y 15 I ,Febru 's, 1946.

JOHN D. SEAVER ET Y a s V It is hereby certified that error appears in the gg'inted specification of :the above numbered patent requiring correction as follows: age 1, second column, line 52,

for the numeral 14 read 12 and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the I Patent Oifice.

Signed and sealed this 24th day of Septemher, A. D. 1946.

',LESLIEFRAZER,Y

' Fir-Assistant Oommiaionn of Patents. 

